class Solution {
public:
    bool hasCycle(ListNode* head) {
        if (!head || !head->next)
            return false;;

        ListNode* slow = head;
        ListNode* fast = head->next;

        while (slow != fast)
        {
            if (!fast->next || !fast->next->next)
                return false;

            slow = slow->next;
            fast = fast->next->next;
        }
        return true;
    }
};